I don't doubt a PRS or Orthopod who trained additionally in Hand would be uber marketable. I'm pretty sure they would be.
I'm talking about a General Surgeon, one who aspired to be a PRS but got sidetracked and never made it out of Hand... What about him? Can you find work as a Hand only surgeon WITHOUT PRS or Ortho under your belt at all?
I'd guess no, since there are PRS and Orthopods out there who can do Hand without fellowship training. So as a backdoor to PRS, I would think, Hand is a terrible idea for a General Surgeon who never found a willing PRS residency.
Burns, on the other hand, seems like it'd be a better deal. Burns still pays well for the Burn Surgeon and the institution that cares for the Crispy Critters. So I would guess that there's plenty of work to go around. Of course there are PRS guys who cover Burns, but I doubt they play with much of the critical care aspect. You'd be an all around, one-stop shop -- a Crispy Surgeon, if you will. You can take care of the drips, fluid management, skin grafts, tub washes, etc...
And then there's the research aspect, no? To beef up one's application for another go at PRS? I'd guess that there's much more publishable data as a Burns fellow than a Hand fellow. That's just my guess without knowing much of either specialty's literature.